1. Field of the Invention
The present invention relates to a low-frequency circuit breaker (low-frequency-current circuit breaker) which is applied to, for example, a wind generator system and cuts off a current having a frequency of about 10 to 20 Hz.
2. Description of the Related Art
Conventionally, as an example of a wind generator system, there is a system configured to convert three-phase alternating-current power generated by a permanent-magnet-type wind generation installation into direct current by a converter to which, for example, an IGBT element is bridge-connected. This current is converted into alternating-current power by an inverter to which, for example, an IGBT element is bridge-connected, and is supplied to an alternating-current load.
In a system configured as described above, a short-circuit accident can be considered to occur in an IGBT element which forms part of a converter due to some reason. When an IGBT element short-circuits, a short-circuit current continues to flow through a diode connected in anti-parallel to the IGBT element. However, at present, there is no circuit breaker for general purpose use, which can cut off a fault current caused by a short-circuit accident, and therefore, development of a circuit breaker for general purpose use is demanded.
A permanent-magnet-type wind generator installation cannot control a field current and hence needs to cut off a current flowing through a line between stators. Unless the fault current is cut off, a properly working semiconductor device such as an IGBT forming part of a main circuit of a converter may be broken.
A synchronous power generator which is used in the permanent-magnet-type wind generator installation has a high internal impedance. There exists a situation that only a short-circuit current which is at most approximately twice greater than a normal current flows even when an IGBT element causes a short circuit Therefore, a current limiting means such as a fuse cannot be used.
Conventionally, an alternating current which is generated by a wind generator installation and flows through an alternating-current path is, for example, a low-frequency current of 10 to 20 Hz. There is a demand for development of a circuit breaker which is advantageous in view of costs, with a simple configuration capable of cutting off the low-frequency current.
At present, a circuit breaker for a direct current is used in a wind generator system. An example of the circuit breaker for a direct current is configured by connecting a thyristor bulb in parallel with a bypass switch such as a gas circuit breaker, as disclosed in Patent Document 1.
Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 7-105789
However, in a direct-current circuit breaker according to Patent Document 1, a thyristor bulb and a gas circuit breaker which form part of the circuit breaker both have large sizes and are disadvantageous in view of costs.
A circuit breaker comprising only a thyristor switch naturally causes conduction loss due to conduction.